1. Field of The Invention
This invention relates to frequency mixing of signals and, more particularly, to a frequency mixer with an extended dynamic range which uses feed forward distortion reduction.
2. Description of Related Art
Frequency conversion of signals is primarily accomplished by a frequency mixer element. The frequency mixer multiplies two or more input signals in the time domain or convolves one or more input signals in the frequency domain. For example, for certain frequency conversion applications, the frequency mixer mixes an input signal having a frequency f1 and a local oscillator signal having a frequency f2. By mixing these signals , the mixer produces first order frequency mixed signal components having the frequencies f1+f2 and |f1xe2x88x92f2| with the amplitude or shape characteristics of the input signal. If frequency upconversion is desired, the lower frequency signal component is filtered out to leave an upconverted signal, and if frequency downconversion is desired, the higher frequency signal component is filtered out to leave a downconverted signal.
The mixing of the input signal and the local oscillator signal, however, also generates intermodulation distortion. In general, intermodulation distortion results from spurious combination frequency components in the output of a nonlinear element when two or more sinusoidal signals form the input. Intermodulation distortion of a complex wave (having multiple frequency components) arises from intermodulation of the components in the complex wave by each other in a nonlinear system, producing waves having frequencies, among others, equal to the sums and differences of the components of the original wave. The power level of the intermodulation distortion generated by a mixer depends upon the input signal power level. Typically, for an increase in input signal power level, the mixer generates a corresponding increase in output signal power level with an even greater increase in the power level of the intermodulation distortion. As such, the highest acceptable power level of intermodulation distortion resulting from the corresponding highest output signal power level defines a boundary for the dynamic range of the mixer. The dynamic range of the mixer can be defined for a given output power level as the difference between the output signal power level and the corresponding power level of the intermodulation distortion. Whether the dynamic range is acceptable depends on the particular application. If a given output signal power level exceeds the dynamic range of the mixer, this usually means that an unacceptable power level of intermodulation distortion is generated by the mixer along with the frequency mixed or converted signal components. Extending the dynamic range of the mixer allows the mixer to produce a greater range of output signal power levels without generating unacceptable levels of intermodulation distortion. For example, in an application where a mixer is operating in a 30 kHz bandwidth, a mixer can have a dynamic range of 100 dB defined by a high output signal amplitude of 0 dBm and a corresponding intermodulation distortion amplitude of xe2x88x92100 dBm. Extending the dynamic range of the mixer occurs by increasing the relative difference between the amplitudes of the output signal and the intermodulation distortion.
A frequency mixer with an extended dynamic range is desirable.
The present invention involves a frequency mixing system which provides an expanded dynamic range when compared to the dynamic range(s) of an individual mixer(s) that make up the arrangement. The frequency mixing system uses a feed-forward arrangement to reduce the distortion emanating from a single mixer due to a signal power level which would result in a frequency converted signal outside the dynamic range of the mixer. For example, the frequency mixing system splits an input signal onto a first path and a second path. On the first path, a first mixer frequency mixes the signal to produce a frequency converted signal with distortion, such as intermodulation distortion. On the second path, the amplitude of the signal is attenuated then frequency mixed by a second mixer to produce a frequency converted signal with a low and/or insignificant level of distortion. The first mixer element produces a frequency converted signal with distortion because the signal on the first path enters the first mixer at a power level resulting in a frequency converted signal outside the dynamic range of the first mixer. As such, the higher power level of the signal into the first mixer creates distortion that emanates from the first mixer along with the frequency converted signal. Since the attenuated signal on the second path enters the second mixer at a lower power level resulting in a frequency converted signal within the dynamic range of the second mixer, the second mixer produces the frequency converted signal with the low and/or insignificant level of distortion. After the signals on the first and second paths are frequency converted, the frequency converted signal and distortion are coupled from the first path and combined with the frequency converted signal on the second path, producing the distortion as the prominent signal on the second path. The distortion on the second path is then combined with the frequency converted signal and distortion on the first path to cancel the distortion on the first path and produce the desired frequency converted signal. Because the frequency mixing system produces the desired frequency converted signal with reduced distortion as compared to the distortion produced if the original input signal would be frequency converted using only the first mixer, the frequency mixing system provides an extended dynamic range.